This invention relates to a process, apparatus and system for removing radioactive radon gas, (radon-222) from potable water supplies, particularly for residential use.
Radon is a colorless, odorless, radioactive gas produced from the natural decay of uranium. In outdoor air, radon is diluted and not recognized as a health hazard. However, when radon gas is trapped indoors in air or water, in sufficiently high concentrations, it can be dangerous. Radon has been shown in several epidemiological studies to be a very potent carcinogen that causes lung cancer in humans. In A Citizen's Guide to Radon OPA-86-004 the United States Environmental Protection Agency (EPA) stated that scientists have estimated that about 5,000 to about 20,000 of the 130,000 lung cancer deaths in the United States in 1986 may have been caused by long term exposure to radon gas. Of these 5,000 to 20,000 deaths, about 500 to 1500 have been attributed to radon from residential potable water supplies. The risk from water borne radon may be higher than the combined risk from all of the other man-made chemical contaminants in residential drinking water.
The EPA is currently developing standards for acceptable levels of radon in public water supplies. The range of values for radioactivity concentration being considered run from 500 pico-curies per liter (pc/1) to 40,000 pc/1. Although the EPA standard for public water supplies may not be directly applicable to private residences, it is likely that the public will accept and regard this standard as the safe level.
Naturally occurring radon in water generally emanates from the radium in bedrock surrounding an underground well and through which the water going to the well flows. This is in sharp contrast to the most commonly known sources of contamination in water, which generally arise from remote point contamination sources such as leaky pipes or improperly disposed of waste materials. As such, methods of purifying water of organic contamination are not clearly applicable to removing radon from water.
There are fundamentally two known methods for treatment of water supplies for radon removal in the prior art: decay storage and spray aeration. Conceptually, the simplest example of decay storage is a large baffled water storage tank. Since radon has a radiological half life of only 3.785 days, simply holding the contaminated water in a storage tank for approximately a month will greatly reduce the radon level. One problem with this type of decay storage is that it requires a large tank which maintains essentially plug flow conditions to prevent backmixing.
Another example of decay storage requires accumulation of the radon on an adsorbent material such as activated carbon. Since the radon decays relatively rapidly, the concentration of radon on the adsorbent bed will initially increase, and then gradually reach an equilibrium with the influent radon concentration in the water. One disadvantage with this type of system is that the adsorbent bed gives off radioactivity as the radon decays, which may present a health hazard. A second disadvantage is that it is difficult to legally dispose of the radioactive carbon filter when it becomes fouled by other water borne contaminants such as iron, sediment or bacteria.
Spray aeration is the only method of aeration which is known in the prior art to be practical on a residential scale for removal of radon. For such spray aeration systems, radon removal efficiencies are reported to be approximately 50% on each spray cycle. Therefore, in order to achieve an overall removal efficiency of 90%, the water must be recycled through the spray aeration device 3 to 4 times. This is disadvantageous in that it requires a spray tank that is relatively large, to provide a sufficient quantity of treated water for use in the home.
In addition, aeration of water using an air stripping column is known in the prior art as a method of removing volatile organic contaminants (VOC) such as trichloroethylene, tetrachloroethylene, and benzene from water. Two general types of air-stripping aeration systems are known. In one type of system, an aeration column of at least fifteen to thirty feet in height is required to remove more than 90% of the organic contaminants in a single pass through the system. An aeration column of such a height is not considered practical for residential use. In the second type of system, a shorter aeration column may be used to remove more than 90% of the organic contaminants in the water, but only with multiple passes of the water through the system. In one such system, contaminated water is taken from an underground well and repeatedly pumped through the aeration system and back into the well. The purified water pumped into the well creates a progressively larger buffer zone against influent organic contamination, which would not be useful in inhibiting further radon contamination as the radon source surrounds the well. Any purified water that was injected back into the well would simply become recontaminated with radon which is a product of the decay of Radium, a naturally occurring element in the bedrock into which the well is drilled.